The protocol by which traditional GSM core network components interact is called MAP (Mobile Application Part). MAP itself is a user of the TCAP (Transaction Capabilities Application Part) protocol, which in turn runs on a SS7 protocol stack (i.e. SCCP over MTP or M3UA or SUA over SCTP).
For those users of OpenBSC who have a need to interoperate with other GSM networks (roaming), the circuit-switched part of OpenBSC has so far relied on the use of a proprietary MSC (by means of the A interface). This closed MSC then talks MAP/TCAP/SS7 to roaming partners.
However, on the GPRS front, we now have OsmoSGSN. However, as opposed to the BSC on the circuit switched side, the SGSN directly interacts with the core GSM network components (both of the home network and the roaming partners).
So in order to run OsmoSGSN interacting with existing HLRs, we need to add a MAP/TCAP/SS7 interface to it. Once this has been done for the SGSN, we of course can do the same for the MSC-part that is currently integrated with OpenBSC.
As there are existing implementations of SCTP (inside the Linux kernel) and SUA (sualibrary), TCAP is the next step in the protocol stack that needs to be implemented. I've been digging into TCAP for the last week(s), and believe I finally understood every part of its operation.
You can think of TCAP as something that facilitates the transport of request-response type transactions over a datagram oriented transport layer. It intends to have lower overhead than a connection-oriented service (e.g. establishing TCP sessions) and supports features such as aggregating multiple user-messages (called components) in a single actual transport-layer message. The idea is to reduce the overhead of message headers and routing.
TCAP is (unfortunately) specified in ASN.1 and thus requires significant effort to parse and construct. Right now I'm using Lev Walkin's asn1c ASN.1 C code generator to generate the parser and constructor functions. The actual TCAP protocol logic is once again implemented in plain C, using the various concepts and utility functions established in OpenBSC (and now part of libosmocore).
The implementation is making good progress and I hope I can do some early testing in about a week from now, and successively move straight to the MAP protocol, implementing at least those parts that we need for GPRS authentication and attach / routing area updates.